This application relates to the refining of steel, and more specifically to the subsurface pneumatic refining of steels which requires the addition of a fuel material in order to obtain the desired tap temperature without encountering slopping.
The term "subsurface pneumatic refining" as used in the present specification and claims is intended to mean a process wherein decarburization of the melt is achieved by the subsurface injection of oxygen gas, alone or in combination with one or more gases selected from the group consisting of argon, nitrogen, ammonia, steam, carbon monoxide, carbon dioxide, hydrogen, methane or higher hydrocarbon gases. The gases may be blown in by following various blowing programs depending on the grade of steel made and on the specific gases used in combination with oxygen. The refining period frequently ends with certain finishing steps, such as lime and/or alloy additions to reduce the oxidized alloying elements and form a basic slag, and addition of alloying elements to adjust the melt composition to meet melt specifications.
Several subsurface pneumatic steel refining processes are known in the art; including, for example, the AOD, CLU, OBM, Q-BOP and the LWS processes. U.S. patents illustrative of these processes, respectively are: U.S. Pat. Nos. 3,252,790; 3,867,135; 3,706,549; 3,930,843 and 3,844,768.
During pneumatic refining, the melt is heated by the exothermic oxidation reactions which take place during the decarburization stage of the refining period. The melt cools quite rapidly during the finishing stage since the additions of lime and alloying elements are endothermic, as well as the fact that no exothermic reactions are taking place.
Subsurface pneumatic refining, commonly referred to in the art as "blowing", normally produces one or more of the following results: decarburization, deoxidation, desulfurization, dephosphorization and degassing of the heat. In order to obtain these results it is necessary: (1) to provide sufficient oxygen to burn out the carbon to the desired level (decarburization), and (2) to provide sufficient sparging gas to: thoroughly mix the deoxidizing additions into the melt (deoxidation), achieve good slagmetal interaction (desulfurization), and assure that low levels of hydrogen and nitrogen will be obtained in the melt (degassing).
Pneumatic refining has two opposing temperature constraints. One is that a sufficiently high temperature be attained by the exothermic reactions to permit the endothermic steps to be carried out while maintaining the temperature of the melt sufficiently high for tapping of the heat. The opposing restraint is that the peak temperature attained in the refining vessel be held below that which will cause excessive deterioration of the refractory lining of the vessel.
Although the present invention is applicable to all of the above-mentioned subsurface pneumatic steel refining processes, for purposes of convenience, the invention will be described and illustrated by reference to the argon-oxygen decarburization process, also referred to for short as the AOD process.
The term, "argon-oxygen decarburization process" as used in the present specification and claims is meant to define a process for refining molten metal contained in a refining vessel which is provided with at least one submerged tuyere, comprising (a) injecting into the melt through said tuyere(s) an oxygen-containing gas containing up to 90% of a dilution gas, wherein said dilution gas functions to reduce the partial pressure of the carbon monoxide in the gas bubbles formed during decarburization of the melt and/or to alter the feed rate of oxygen to the melt without substantially altering the total injected gas flow rate, and thereafter (b) injecting a sparging gas into the melt through said tyere(s) wherein said sparging gas functions to remove impurities from the melt by degassing, deoxidation, volatilization, or by flotation of said impurities with subsequent entrapment or reaction with the slag. The process normally has the oxygen-containing gas stream surrounded by an annular stream of protective fluid which functions to protect the tuyere(s) and the surrounding refractory lining from excessive wear. Useful dilution gases include: argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide or steam; argon is preferred. Useful sparging gases include argon, helium, nitrogen and steam; argon being preferred. Useful protective fluids include argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam or a hydrocarbon fluid; argon again is preferred.
During refining, the temperature of the melt is influenced by those factors that constitute head losses and those that constitute heat gains. Heat is required:
(1) to raise the temperature of the melt from its charge temperature to its tap temperature,
(2) to dissolve lime and other constituents of the slag,
(3) to dissolve any alloy, scrap or other additions made during refining, and
(4) to make up for the heat lost by the melt to its surroundings during the overall refining period (i.e. during inert gas stirring, blowing, reduction and turndowns).
Heat is supplied during the refining period only by the exothermic reactions which take place during refining. These include the oxidation of carbon, silicon, aluminum and any other metallic constituents in the melt, such as, for example, iron, chrome and manganese. If after refining, the melt temperature is insufficient to achieve the desired tap temperature, it is common practice to reblow the heat with oxygen, thereby generating heat by the oxidation of carbon and metallic elements in the melt. Such reblowing, however, is undesirable because it takes additional time, requires the use of additional oxygen, silicon and lime, and causes undesirable oxidation of metallic elements in the melt, all of which produce inefficiency in the overall refining operations, and adversely affect the quality of the metal.
One way of avoiding the above-mentioned problem is disclosed by Choulet and Mehlman in U.S. Pat. No. 4,187,102 issued on Feb. 5, 1980. The method described therein comprises the addition of fast and slow oxidizing elements to the melt (such as aluminum and silicon, respectively) before starting the injection of refining oxygen. The heat provided by the oxidation of these elements must be sufficient to leave the temperature of the melt at the end of the refining period at least equal to the desired tap temperature, but not so great as to cause excessive refractory deterioration. While satisfactory in many cases, the process disclosed by Choulet and Mehlman may cause severe "slopping" in some instances.
"Slopping" is a metallurigcal phenomenon common to pneumatic refining of metals wherein the slag-metal emulsion formed above the melt being refined surges up and out the open mouth of the refining vessel. Slopping is not only detrimental to yield, but dangerous to workers who are near the vessel.
It has been found that the following factors increase the tendency of a heat of steel to slop during AOD refining:
1. High rates of carbon monoxide evolution. PA1 2. High gas (argon and/or oxygen) blowing rates. PA1 3. Small freeboard volume in the refining vessel. PA1 4. Formation of a slag-metal emulsion.